Control system



July 12, 1938. c SCHIEBELER 2,123,,?-%7

CONTROL SYSTEM Filed Feb. 25, 1937 Inventor 2 Carl Schiebeler,

His Attorney Patented July 12, 1938 PATENT OFFICE CONTROL SYSTEM CarlSchiebeler, Berlin-Charlottenburg,

assignor to General Germany,

Electric Company, a corporation of New York Application February 23,

1937, Serial No. 127,240

- In Germany February 29, 1936 16 Claims.

This invention relates to control systems, more particularly to systemsfor controlling the operation of electric motors, and it has for anobject the provision of a simple, reliable, and improved system. of thischaracter.

More particularly, the invention relates to control systems foralternating current. induction motors connected to drive hoists and thelike, which are subjected at times to overhauling loads,

and a. further object of the invention is the provision of means forcontrolling the motor to lower an overhauling load.

A further object of the invention is the provision of means forcontrolling an alternating current motor to lower an overhauling load atany one of a plurality of subs'ynchronous speeds.

In carrying the invention into effect in one form thereof, a masterswitch is provided together with a mechanical brake for controlling gothe motor speed. Mechanism including an electric motor is provided foroperating the brake, and means driven by the main driving motor supply avoltage to the brake operator motor that varies with variations in thespeed of the main 2.] driving motor. A variable speed driving mechanismis interposed in the driving connections between the main driving motorand the voltage supplying means driven thereby, and means actuated inresponse to operation of the master switch 30 are provided for varyingthe drive ratio of the variable speed drive.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accom- 35panying drawing, the single figure of which is a simple, diagrammaticillustration of an embodiment of the invention.

Referring now to the drawing, an electric motor I0 is mechanicallyconnected to the winding drum 40 ll of a hoist, or similar apparatus,which under certain operating conditions, may impose an overhauling loadon the motor. The motor ID is illustrated as an alternating currentmotor of the wound rotor induction type. Power is sup- 45 plied to thismotor from a suitable source represented by the three supply lines l2 towhich the primary winding of the motor is arranged to be connectedthrough conductors I3 by means of suitable reversing switchingmechanismillustrated so as a pair of electromagnetically operated contactors Hand [5. When contactor II is closed, the motor I0 is energized forrotation in such a direction as to lower the load, and when contactor I5is closed, motor I0 is energized for 55 rotation in the hoistingdirection. The drum of a mechanical brake I6 is mounted on the motorshaft ll. Suitable means, illustrated as a spring l8, are provided forbiasing the braking shoes We, I61, to the braking position, andoperating means H) are provided for releasing the brake 5 shoes. Thisbrake operating means is actuated by an electric motor 28 and ispreferably in the form of an electrohydraulic operating mechanism,comprising a piston immersed within a cylinder containing oil or othersuitable fluid lo and connected by means of a connecting rod 2i to thearm of the brake shoe actuating mechanism, and a centrifugal pumpimpeller within the cylinder driven by the electric motor for producinga liquid pressure against the piston of sufi5 ficient magnitude torelease the brake shoes ifii, lit against the bias of the spring is.

The contactor 22 is provided for connecting the motor 20 of thehydraulic operating mechanism to the source E2 to efiect release of thego brake. Another contactor 23 is provided for connecting the motor 20to a source of voltage 24 that varies with variations in the speed ofthe main driving motor Hi. This source of voltage 2t is illustrated as afrequency changer having its 25 primary windings connected to the sourcel2 through conductors 25 and upper contacts of the lowering contactorl4, and having its secondary winding connected through contactor 23 andconductors 26 to the motor 20 of the brake operating'3o mechanism. Thedrive shaft of the frequency changer 24 is connected through a suitablevariable speed driving device to the drive shaft of the main motor l0.While this variable speed driving device 21 may be of any suitable type,it is illustrated as a Reeves pulley. Briefly, a Reeves pulley comprisesa pair of tapered pulleys 2h, 21:, and a second pair of tapered pulleys27, 21d connected by an endless belt 28. The pair of tapered pulleys21a, 21s are slidably mounted on the shaft 0 28 and similarly, the pairof pulleys 21c, 27s are slidably mounted on the shaft 30. Thus when thepulleys of each pair are separated the maximum distance, their effectivediameter is a minimum and likewise, when the pulleys are closesttogether, their effective diameter is maximum. Thus, the tapered pulleys21a and 21b, which are illustrated as being separated from each otherthe maximum distance, present the minimum eflective diameter to the belt28 and conversely, the pair of tapered pulleys 21c, 21d which areillustrated moved close together present maximum efiective diameter tothe belt 28. By separating one pair of pulleys while moving the otherpair of pulleys together, a smooth, stepless variation in the driveratio between the shafts'ilQ and 80 is obtained, and for this purpose apair of pivoted arms 3 l, 32 actuated by a screw shaft are provided.When the screw 33 is rotated in one direction, the pulleys Zia, 21s areseparated and the pulleys 2 11s, 27s are moved toward each other, andwhen the screw 33 is rotated in the reverse direction, the reverseaction takes place.

A multiposition reversing type master switch 3 3 is provided forcontrolling the starting, stopping, speed, and direction of rotation ofthe motor iii. A variable resistance 85 is included in the secondarycircuit of the motor iii. When an alternating current motor is takingpower from the line and driving a load, its speed can be controlled byvarying the amount of secondary resistance. Master switches such as theswitch 36 are usually provided with a number of segments and fingers forvarying the resistance in the secondary circuit of the motor. Suchsegments and fingers have been omitted from the drawing in the interestof simplicity.

It is well known that if a wound rotor induction motor is overhauled byits load, it develops no braking torque except at speeds abovesynchronism. Increasing the secondary resistance when the motor is beingoverhauled serves only to increase the motor speed. Increasing thesecondary resistance decreases the speed of a wound rotor inductionmotor only when the motor is taking power from the line to drive theload. Thus by utilizing the mechanical brake it to impose a motoringload on the motor when its regular load becomes overhauling, the speedof the motor can be controlled by means of the master switch at and thesecondary resistance 35 controlled by the master switch, and the motorthus caused to operate at subsynchronous speeds.

In order that the brake it may be utilized to regulate the speed of themain motor it, i. e., to maintain the speed of the main motor itsubstantially constant at any one of a plurality of desiredsubsynchronous speeds. -it is necessary that the brake operatingmechanism should be controlled by variations in the speed of the mainmotor. Consequently, when it is desired to lower on overhauling load,the motor 29 of the brake operating mechanism isconnected to thefrequency changer'fid driven by the main motor iii. The frequency andmagnitude ofthe voltage at the slip rings of the frequency changer varyinversely with the speed of the main motor. Thus, at zero sped of themain motor the frequency changer 2 operates as a stationary transformer,and the frequency and magnitude of the slip ring voltage are maximum.Conversely, at synchronous speed of the main motor the magnitude andfrequency of the slip ring voltage become zero, assuming thevariablespeed drive adjusted for one to one driving ratio. Since the speed ofthe operator motor 20 depends upon the frequency of the applied voltage,it will operate at maximum speed when the frequency changer 2A is atzero speed, and will operate at zero speed when the frequency changeris'running at synchronous speed. Thus, the speed of the brake operatormotor 26 is seen to be inversely proportional to the speed of the maindriving motor Ill.

The lifting force characteristic of the electrohydraulic brake operatingmechanism is not variable over the entire *speed range of the operatormotor 20. For example, this characteristic may be such that the brake isfully released at 80 per cent of full speed on motor 26] and at 50 percent speed the brake may be applying 6d per cent of full raisingtorque.This is due to the law governing centrifugal pump which is that thepressure exerted varies as the square of the impeller speed. Thus itwill be seen that the range of speeds of the main motor ill betweenwhich the brake is set or fully released is a narrow one, if frequencychanger 2G is geared l to l to motor 9E9. Consequently, in order thatthe brakes may be utilized to cause the main motor ill to produce amotoringtorque for a plurality of subsynchronous speeds over a widerange, it is necessary that the frequency changer 2 3 should operate atsomewhere near the same speed for each subsynchronous speed of the maindriving motor it; This is accomplished by varying the drive ratio of thevariable speed drive in response to actuation of the master switch 36.To this end, the operating lever of the master switch is connected tothe screw shaft 38 of the variable speed drive. Although any suitableconnecting means, mechanical or electrical, may be utilized for thispurpose, electrical self-synchronous motion transmitting meansare-preferred.

Such means comprise an electrical motion transmitting device 36mechanically connected to the master switch shaft 3?, and a motionreceiving device 35 connected to the screw shaft 33 of tlie variablespeed device 2?. Each of these devices may be physically similar to awound rotor induction motor. The stator windings of them devices areconnected through conductors 39 and master switch 136 to the alternatingcurrent supply source l2, and the rotor windings of both devices areconnected together by means of conductors lu. When thus connected andthe rotor member of the transmitting device rotated from one position toanother, the rotor of the receiving member will rotate to acorresponding position.

For any given load suspended from the hoist drum-ii, the braking torquerequired to obtain any given speed reduction will be the sum of thetorque produced by the load'plus the motor torque delivered by the motorat that speed when operating on the speed torque curve corresponding tothe resistance then in circuit. With this in mind, initial adjustmentsare made so that the voltage and frequency from the frequency changerare such that the braking torque produced by the brake will besufilcient to reduce the speed of the main motor to a value at which itwill be delivering motor torque instead of regenerative torque.

ments and their arrangement in the system, the operation of the systemwill readily be understood from the following detailed description.

If it is desired to lower the load, the master switch at is operatedfrom the central or ofi position in which it is shown to the left-handor lowering position. As the master switch is operated through itssuccessive positions, the rotor of the transmitting device 38 is rotateda corresponding amount and, the rotor of the receiver 3% is likewiserotated a corresponding amount. The direction of rotation of the screw33 produced by operation of the master switch to the left-hand positionsis such that the pulleys 2%, 2h, are forced together and the pulleys27c, 2% are separated. Thus, when the master switch 34 is in its fifthor full-speed lowering position, the frequency changer 2% is operatingat approximately 20 per cent synchronous speed.

As the master switch passed through its first lowering position, anenergizing circuit was established for the operating coil of thelowering contactor l4. This circuit is traced from the upper supply linel2 through conductor 4|, finger 42, segments 43 and 44, finger 45,conductor 46, operating coil of contactor l4 and thence by condoctor 41to the lower supply line l2. Contactor i 4 then closed its contacts inresponse to energization and connected the motor ID to the source "i2for rotation in a direction to lower the load. Contactor M in closingalso connected the terminals of the frequency changer 24 to the sourcei2. Simultaneously, an energizing circuit was established for theoperating coil of contactor 23 that is traced from the power segment 43to segments 48, finger 49, operating coil of contactor 23 to the lowerside of the supply source l2. Contactor 23 closed in response toenergization and connected the slip rings of the frequency changer 24 tothe operator motor 20. At the instant the master switch 34 is moved toits first position, the main'driving motor I0 is of course stationaryand likewise, the frequency changer 24 is stationary. Therefore, thefrequency changer acts as a stationary transformer, and voltage ofmaximum frequency is supplied to the operator motor 20, thereby causingthis motor to rotate at maximum speed and produce a force to release thebrake against the tension of the spring I 8.

In actual operation, the operator usually moves the master switchquickly in one motion from the off position to the full on position. Asthe master switch passes through its successive lowering positions, thesecondary resistance 35 of the main driving motor i0 is short-circuitedin steps usually timed automatically and the motor accelerated to fullspeed. Simultaneously, the speed of he frequency changer 24 is beingreduced and as long as the speed of the motor Ill does not exceed thespeed at which it is designed to run on any position of the masterswitch, the frequency of the voltage supplied from the frequency changer24 to the brake operator motor 28 is sufllcient to maintain the brakereleased. On the fifth or full-speed position of the master switch. theenergizing circuit of the contactor 23 is interrupted. and thiscontactor opens its contacts to disconnect the brake operator motor 20from the frequency changer 24. Substantially simultaneously. anenergizing circuit is completed for the operating coil of contactor 22which closes to connect the brake operator motor to the supply sourceI2. As a result of this, voltage of full-line frequency is supplied tooperator motor 20 which thereforeoperates at substantially synchronousspeed and maintains the brake released as long as the master switch 34is maintained in its full-speed lowering position.

.ing that it is desired If the load should overhaul the main motor i0,

the speed of the latter will increase above synchronous speed. Assumingthat the operator desires to lower the load at a subsynchronous speed,it therefore becomes necessary for him to notch the master switch handleback to one of the subsynchronous speed positions. Assumto lower theload at a subsynchronous speed corresponding to the third loweringposition of the master switch, the handle of the master switch isnotched back to the third position. As a result of this movement,

such portions of the secondary resistance 35 are inserted in thesecondary circuit of the main motor 50, contactor 22 opens and 23 closesand simultaneously, the drive ratio of the variable speed device 2'! isincreased. If, after this movement has been completed, the speed of themain motor i0 is above the speed at which it should operate on the thirdpoint of the master switch, the speed of the frequency changer 24 willbe h gher and its slip ring frequency and voltage will be lower than itshould be if the motor i 0 were operating at the correct speed.Conseouently, the speed of the brake operator motor 28 Will becorrespondingly reduced, and the spring i8 will apply the brake shoessuiliciently to reduce the speed of the motor ill to the correct value.Likewise, if the speed of the motor I 0 is less than the correct value,the frequency of the voltage supplied from the frequency changer 24 tothe operator motor 20 will increase and thereby reduce the braking forceof the brake a corresponding amount. It will thus be seen that the speedof the motor l0 isregulated to a desired subsynchronous value for eachposition of the master switch in the lowering direction.

It will be observed that for all hoisting positions, the contactor 23 isopen and the motor 20 of the brake operating mechanism disconnected fromthe slip rings ofthe frequency changer whilst the contactor 22 is closedso that the motor 2G is connected to the supply source I2. As a resultvoltage of full-line frequency is supplied to the operator motor 20, andthe brake is maintained released'for all hoisting positions.

Although in accordance'with the provisions of the patent statutes, thisinvention is described as embodied in concrete form, it will beunderstood that the apparatus and connections shown are merelyillustrative, and that the invention is not limited thereto, sincealterations,

and modifications will readily suggest themselves to persons skilled inthe art without departing from the true spirit of the invention or fromthe scope of the annexed claims.

What I cla m as new and desire to secure by Letters Patent of the UnitedStates is: Y

1. A control system for an electric motor subject to overhauling loadscomprising a speed controller for the motor. a mechanical brake for saidmotor. means mechanically driven by said motor for controlling thebraking force of said brake in accordance with changes in the speed ofsaid motor, and means for varying the speed ratio between said motor anddriven means in response to operation of said speed controller.

2. A control system for an electric motor sub- .iect to an overhaulingload comprising in comb nation a speed controller for said motor, amechanical brake for said motor, operating means for said brake, asecond motor for actuating said operating means, means driven by saidfirst motor for controlling the speed of said v secondfmotor thereby to'vary the force of said of said motor, a mechanical brake for saidmotor, operating mechanism for said brake, an alternating current motorfor actuating said operating mechanisxn, adynaznoelectricchine-connected to said alternating current motor anddriven by said first motor for causing said brake operating mechanism tovary the braking torque in accordance with variations in the speed oz?said first .motor, a variable speed device between said first motor anddynamoelectrtc machine, and means responsive to actuation of said masterswitch for controlling said variable sweet device.

A control system 5. A control system comprising in combination, a mainmotor, a rnuitipoint master switch for controlling the speed of saidmotor, a mechanical brake for said motor, operating means for controiiintire application of said brake, said operat-.

ing means comprising an alternating current motor, an alternatingcurrent machine driven by said main motor for supplying an alternatingvoltage to said alternating current motor, thereby to vary the brakingtorque of said brake in accordance with variations in the speed of saidW main motor, a variable speed device included in t wi main motor andelectric the driving connectionsioetween said main motor and alternatingcurrent machine, and means actuated toy operation of said master switchfor varying the drive ratio oi said variable speed drive in accordancewith the operation of said master switch.

6. A control system comprising in combination a main motor, a reversingtype rnuitiposition master switch for controlling the speed anddirection oi said motor, a mechanical bra-ice for said inc-tor,operating mechanism for said iiraire, operating mechanism. including aelec motor, an electric machine driven by said 1 motor tor supplying tosaid second motor a voltage varying with the of said motor thereby tocontrol the braking torque E brake in accoruse with variations in thespeed oi said n tor, ro n responsive to actua tion at said rn sterswitcn tor one .rection of rotation for establishing connections beenelectric machine and second sive to actuation of said in conceit-edirect-ion for int tions, a var isle sneeo drive K machine sponsive toactuation of in varying the ratio of said drive o once with the user ton of l i. A contro syst comprising i a main alt current motor, a innlpoint master switch for controlling the of said motor, a mechanicalbrake 502' s ld. motor, one at= ing means for said inclurn a second al=ternating current motor, frequency changer g for a main driving niotorcomprising a master switchior controlling the speed of said viceincluded in the driving connections between said main motor and saidfrequency changer, and means responsive to actuation of said masterswitch for varying the drive ratio of said variable speed device wherebysaid frequency changer is caused to operate at predetermined percentagesof its synchronous speed for selected speeds of said main mote 8. Amotor control system for hoists and the like comprising in combinationan alternating current main driving motor subject to overhauling loads,a multipoint reversing type master switch for controlling the speedand-direction of rotation of said motor, a mechanical brake tor saidmotor, operating'nieciianism for said brake including a secondalternating current electric motor, means responsive to actuation ofsaid master switch in the looisting direction for energizing said secondmotor to release said brake, a frequency changer driven by said mainmotor for producing a voltage of a frequency varying inversely withvariations in the speed of said main motor, means responsive toactuationof said master switch in the lowering, direction for connecting saidsecond motor, to said frequency changer to provide for varyingttielorainng torque or said brake in accordance with variations in the speedof said main motor, a variable speed drive device in the drivingconnections between said main motor and frequency changer, and meansresponsive to actuation of said master switch for varying the driveratio or" said variable speed device in accordance with operation ofsaid master switch.

9. 5%. control system corner lg in combination, -incai brake thereswitenor coroiling speed 'ro-h au c Brake operator havdriving main motlo:hating tor a voltage 3! with var or, varia-M connections lace sf-1G3changer,

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